A recording type DVD (Digital Versatile Disc) has become widespread as a large capacity optical disc. In the recording type DVD, spiral grooves are engraved on the recording surface of a disc as tracks for recording an information pattern. In a DVD-R disc, the grooves are formed to slightly meander according to a carrier signal composed of a sine wave. In a recording and reproducing apparatus for DVD, a light beam emitted from an optical head is positionally controlled so as to converge on a groove acting as a recording track. Further, the number of rotation of an optical disc is controlled such that a light beam carries out scanning on a recording track at a predetermined linear speed. To control the rotation of the optical disc easily, a carrier signal reproduced from the meandering of a track is used. Since the grooves meander at a predetermined space frequency, the linear speed can be kept constant by controlling the number of rotation so that the frequency of a reproduced carrier signal is made constant. When the linear speed is kept constant, an information pattern which is recorded in synchronization with a recording clock kept to a predetermined frequency is formed on grooves as patterns having a predetermined line density.
In the DVD-R disc, digital information including address information is represented by prepits disposed between grooves as emboss pattern. The prepits are disposed in addition to the meandering of the grooves. When data is recorded to a particular address on a disc, a recording start position is controlled by decoding the information of the prepits.
An address information showing a position on a disc can be represented by a method of modulating the meandering phase of grooves as in DVD+R, in place of a method of representing it using prepits as in DVD-R. In the DVD+R, a signal, which is caused to represent digital information including address by inserting a sine wave whose phase is locally different 180° from a sine wave carrier signal into the carrier signal, is used as a wobble signal, and grooves are meandered according to the signal. Address information can be obtained by decoding a reproduced wobble signal.
A recording and reproducing method of related art and the functions of the respective blocks in an optical disc apparatus will be explained below based on a block diagram of an optical disc apparatus shown in FIG. 13.
A light beam emitted from an optical head 2 is converged on an optical disc 1, and the light reflected therefrom is received by a photodetector (not shown in FIG. 13) divided into two portions in a direction along a groove. A signal which changes along the meandering of the groove can be obtained by calculating the difference of current outputs obtained from respective detectors by a regenerative amplifier 3. However, the light beam converged on the optical disc 1 also partly covers adjacent grooves in addition to a target groove.
FIG. 14 conceptually shows the grooves on the optical disc and a light beam converged thereon. A light beam 201 emitted from the optical head 2 is converged on the optical disc 1, and although the greater part of the components of the light beam 201 intensively distribute within an circle shown by a dotted line, a part of the component also distribute to the outside of circle. Accordingly, a part of the reflected light includes the light in the vicinity of the circle. The original wobble signal obtained from the regenerative amplifier 3 is a signal in which a crosstalk component and noise corresponding to the meandering of the adjacent grooves are mixed. However, conventionally, the crosstalk and noise are treated as a disturbance, and a wobble clock synchronized with the original wobble signal is obtained by a wobble clock extraction circuit 6 based on the output from the regenerative amplifier. A circuit such as a PLL (Phase Locked Loop) synchronization circuit provided with a high frequency stability by absorbing a phase variation due to noise is used as the wobble clock extraction circuit 6.
The address information is extracted by using a wobble signal decoding circuit 5 on the basis of the original wobble signal and wobble clock. The wobble signal decoding circuit 5 includes, for example, a band-pass filter 501, a sampling circuit 502, a synchronization circuit 503, and an address decoder 504. The band-pass filter 501 has a pass band in the vicinity of wobble frequency and removes an out-of-band noise component from the original wobble signal, the sampling circuit 502 samples the output from the band-pass filter in synchronism with the wobble clock, the synchronization circuit 503 binarizes and synchronizes the output from the sampling circuit 502, and the address decoder 504 decodes the output from the synchronization circuit 503 and extracts the address information.
When the number of rotation of a spindle motor 13 is controlled using a spindle control circuit 12 so that the frequency of the wobble clock obtained by the wobble clock extraction circuit 6 is made constant, the scanning speed of a light beam is kept to an approximately constant linear speed. A disc system control circuit 10 creates a record pattern in synchronism with a recording clock kept to a predetermined frequency on the basis of the address information obtained from the wobble signal decoding circuit 5, and the intensity of the light beam is modified through a recording control circuit 11, thereby an information pattern can be formed on the optical disc 1 at a predetermined line density.
Further, the disc system control circuit 10 detects the total amount of the light reflected from the optical disc 1 by the current output of the photodetector output from the optical head 2 through a regenerating (reproducing) control circuit 16 on the basis of the address information obtained from the wobble signal decoding circuit 5 and reads the information pattern on the optical disc 1.
A clock synchronized with the wobble clock may be created and used by a multiplying circuit (integral multiplication circuit) in place of using an oscillator having a fixed frequency, as a recording clock creation (generation) circuit. Although the oscillator having the fixed frequency is excellent in a frequency stability, it does not follow to the variation of the linear speed caused by the decentering of the optical disc. Accordingly, it is difficult for the oscillator to suppress the accuracy of an information pattern forming position to equal to or less than an amount of phase variation. In contrast, when a recording clock is created by integral multiplication of the wobble clock, since an information pattern can be recorded following to the linear speed detected from the wobble frequency, a high positioning accuracy can be obtained.
When grooves meander at a predetermined space frequency as in DVD-R and DVD+R, phases of the meandering do not match with each other in grooves adjacent to each other and are gradually dislocated. Accordingly, the variations of the amplitude and phase of a frequency corresponding to the difference between the meandering frequency of the groove corresponding to a track to be processed and the meandering frequency of grooves adjacent to the groove appear to the original wobble signal with which a crosstalk is mixed. When it is intended to directly extract a wobble clock from an original wobble signal by using the PLL as in the example of the related art, phase dislocation also appears in a reproduced wobble clock due to a crosstalk.
When it is necessary to produce information including an address by decoding the meandering phase of grooves as in DVD+R, a problem arises in that a decoding error is liable to occur because the phase of a wobble clock which must act as a phase determination standard is dislocated.
Patent Document 1 discloses an example of a decoding apparatus for stably reproducing an address even if a wobble signal is deformed by a crosstalk component and the like.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2004-134009 (paragraph Nos. [0034], [0035], FIG. 8)